Short-wavelength Free-electron Lasers Research Articles

A proposed superconducting photoemission source of high brightness

We describe the design of an electron injector for the production of a bunched cw electron beam of high brightness, low energy spread, and potentially high beam power. Electron beams of this kind are required for high efficiency and short wavelength free electron lasers. The injector consists of a superconducting reentrant cavity housing a photoemission cathode evaporated on a superconducting substrate. The cathode is illuminated with short light pulses of a mode-locked frequency doubled Nd: YAG laser. The experimental layout of the “superconducting photoemission source” is described together with its components: the photocathode preparation chamber, the cavity, the cryogenic setup, and the beam analysis system. The conceptual beam parameters are discussed and first results of calculations on beam dynamics using a particle-in-cell computer code are given.

Compact short-wavelength free-electron laser.

A new nonclassical model is proposed for the radiation from electrons skimming over and colliding at grazing incidence with a conducting diffraction grating. Radiation originating from induced surface currents and bremsstrahlung is amplified as it passes through spatially modulated wave functions above the grating. The predicted magnitude and dependence on electron beam thickness agree with experiments at visible wavelengths and suggest the possibility of a compact x-ray laser.

Testing the photon–photon sector of quantum electrodynamics with free-electron lasers

Bright short-wavelength free-electron lasers with a high repetition rate may allow for experimental tests of the photon–photon sector of quantum electrodynamics. Since the relevant cross sections are optimal for photons with an energy of the order of the electron rest mass, Compton backscattering of the free-electron-laser electron beam is suggested to generate intense photon beams in this energy range that are then brought to head-on collision. The rates for photon–photon scattering, electron–positron pair creation by two and three photons, and harmonic production in Compton backscattering are estimated. Chances of experimental verification may be highest for pair creation by two photons (the Breit–Wheeler process).

Plasma focus wiggler for free-electron laser applications

A magnetically stabilized plasma focus with controlled modulations has been demonstrated for the first time. A 3-mm-diam plasma focus with two wiggler periods of 1 cm each was kept stable for about 30 ns. Similar configurations with shorter periods could be used as intense wigglers for short-wavelength free-electron lasers.

Relativistic quantum dynamics and quantum noise in short-wavelength free electron lasers

A novel relativistic quantum theory of the free electron laser is formulated using the Dirac equation. It is shown that the theory has the correct classical limit. It is predicted that for two-stage free electron lasers, quantum noise can be comparable to the classical noise due to the thermal spread of the beam, or the random initial distribution of electrons in phase.

Optical guiding measurements on the Mark III free electron laser oscillator

It has been predicted for several years that light is focused (optically guided) as well as amplified by the electron beam in a free electron laser (FEL). The degree of focusing depends strongly on both electron beam and optical beam characteristics. In an FEL oscillator the degree of focusing varies with intracavity optical power during the macropulse. We report the first direct measurements of the evolution of transverse optical mode size and shape between small signal and saturation in a short wavelength (Compton regime) FEL oscillator. The mode measurements on the Mark III FEL oscillator are shown to be consistent with theory, requiring both refractive and gain contributions to guiding.

The Los Alamos photoinjector program

Free electron lasers (FELs) require electron beams of high peak brightness. In this presentation, we describe the design of a compact high-brightness electron source for driving short-wavelength FELs. The experiment uses a laser-illuminated Cs3Sb photoemitter located in the first rf cavity of an injector linac. The photocathode source and associated hardware are described. The doubled YAG laser (532 nm), which is used to drive the photocathode, produces 75 ps micropulses at 108 MHz repetition rate and peak powers of approximately 300 kW. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a 4 ps resolution streak camera. Present experiments give the following results: micropulse current amplitudes of 100 mA to 400 A, beam emittances ranging from 10 π mm mrad to 40 π mm mrad, an energy spread of ± 3%, and peak current densities of 600 A/cm2 The design of experiment has now been changed to include a separately phased rf cavity immediately following the first cavity. This modification enables us to study the effects of phasing with the possibility of improving the injector performance. Also, this change will improve the vacuum conditions in the photoelectron source with a consequent improvement in lifetime performance. A brief discussion on the possible applications of this very bright and compact electron source is presented.

Relativistic quantum dynamics and quantum noise in short-wavelength free-electron lasers.

A novel relativistic quantum theory of the free-electron laser is formulated using the Dirac equation. It is shown that the theory has the correct classical limit. It is predicted that for two-stage free-electron lasers, quantum noise can be comparable to the classical noise due to the thermal spread of the beam or the random initial distribution of electrons in phase.

フリーエレクトロンレーザー

Free electron Lasers (FEL) have advantages of wide tunability, high efficiency and no lasing material which imply the high potentiality for various applications. However very low emittance of electron beam and high accuracy of wiggler magnetic field is required to obtain FEL oscillation. Recent progress of accelerator technology overcomes these issues and makes possible to obtain high power short wavelength FEL. We present the principal of FEL, its properties and system briefly. The applications for various field is also discussed.

Hybrid undulator design considerations

Permanent magnet undulators have become an important part of short-wavelength free electron lasers. The properties and tolerances of the materials employed in undulators become a major issue as the requirements for maintaining interaction strength become more stringent with shorter wavelength FELs. The engineering approach, the magnet specifications, pairing, sorting, arrangement, and field and mechanical tolerances form critical parts of the construction of an acceptable magnetic field configuration with real magnets. This is particularly true as the length of the undulator increases and the photon and undulator wavelengths decrease. This paper will discuss several of the modeling, design, and engineering aspects employed to obtain the required high quality magnetic properties with a hybrid permanent-magnet undulator. The impact of these design choices on the recently completed tapered hybrid undulator (THUNDER) and its performance are also discussed.

Optical guiding in a free electron laser

The coherent interaction between an optical wave and an electron beam in a free electron laser (FEL) is shown to be capable of optically guiding the light. The effect is analyzed using a two-dimensional approximation for the FEL equations, and using the properties of optical fibers. Results of two-dimensional (cylindrically symmetric) numerical simulations are presented, and found to agree reasonably well with the analytically derived criterion for guiding. Under proper conditions, the effect can be large and has important applications to short wavelength FELs and to directing intense light.

The axial velocity spread acceptance of free electron lasers and its application to X-ray FEL design

We present various criteria for the definition of axial velocity spread acceptance of free electron lasers (FEL), and provide analytical approximations, numerical calculations and design curves for this parameter in the low, high, and intermediate gain regimes. This parameter is used to calculate the energy spread acceptance, emittance acceptance and other e-beam quality parameters. A special emphasis is put on the design considerations of short wavelength (X-ray) FELs. For this reason, the parameter calculation in this work is limited to the tenuous beam regimes only, and parameters are calculated for intermediate and moderately high gain regimes, which correspond to the low quality factor of X-ray resonators in the present state of the art.

Optical guiding in a free-electron laser.

By use of two-dimensional approximations for the equations that describe a high-gain free-electron laser (FEL) amplifier, and the properties of optical fibers, it is shown that the coherent interaction between the light and the electron beam in an FEL can optically guide the light. In the exponential-gain regime, the FEL performance in the presence of strong diffraction can be simply described by a cubic equation for the complex gain and the dispersion relation for an optical fiber. The phenomenon of optical guiding is illustrated with two-dimensional numerical simulations. The phenomenon has applications to short-wavelength FEL's, to directing of intense light, and to bending of x rays.

LINAC TECHNOLOGY FOR FREE-ELECTRON LASERS

Electron linear accelerator technology for high-power, short-wavelength free-electron lasers is reviewed.

Theory of a Free-Electron Laser

A classical, linear theory is given for the gain of a short-wavelength free-electron laser. Waves propagating along the relativistic beam are unamplified when the beam is cold. Two of the six geometric optics modes have phase velocities

Renormalizability in LSZ pertubation theory: O. Steinhamnn, Courant Institute of Mathematical Sciences, New York University, New York, New York

Short wavelength Free-Electron Lasers (FELs) are among the newest light sources available to scientists to probe a wide range of phenomena, with chemical, physical and biological applications, using soft and hard X-rays. These sources include the currently most powerful hard X-ray light sources in the world and are characterized by extremely high powers and high transverse coherence, but the first FELs had, and many still have, reduced longitudinal coherence. Now it is possible to achieve good longitudinal coherence (narrow bandwidth in the frequency domain) and here we discuss and illustrate a range of experiments utilizing this property, and their underlying physics. The primary applications are those which require high resolution (for example resonant experiments), or temporal coherence (for example coherent control experiments). The currently available light sources extend the vast range of laboratory laser techniques to short wavelengths.